Method and Device to Determine the Nitrogen Oxide-Storage Capability of a Catalytic Converter of a Vehicle

ABSTRACT

A method to determine a nitrogen oxide storage capacity of a catalytic converter of a vehicle in which a concentration of nitrogen oxides is measured in an exhaust gas downstream of the catalytic converter includes, in at least one first step, setting a concentration of nitrogen oxides in the exhaust gas at which the catalytic converter absorbs nitrogen oxides, and in at least one second step, setting a concentration of nitrogen oxides in the exhaust gas at which a desorption of nitrogen oxides by the catalytic converter takes place. The nitrogen oxide storage capacity of the catalytic converter is determined by considering a behavior of the catalytic converter at least during the desorption of nitrogen oxides.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method and a device to determine the nitrogenoxide storage capability of a vehicle's catalytic converter. This methodmeasures the nitrogen oxide concentration in the exhaust gas downstreamof the catalytic converter.

It is known from the state of the art that the performance of acatalytic converter having nitrogen oxide storage capacity can bedetermined by means of specially introduced measures with respect to areduction of hydrocarbons (HC), carbon monoxide (CO), and nitrogenoxides (NOx) in the exhaust gas of the combustion engine of a vehicle.This includes for instance especially introduced exothermic processes,in other words, heating the catalytic converter, or an enrichment step,in other words, operating the combustion engine by means of an enrichedair-fuel mixture.

Furthermore, DE 198 52 240 A1 describes a method to monitor a NOxstorage catalytic converter whereby the NOx storage efficiency of thecatalytic converter from the NOx exhaust gas concentration is identifiedbefore and after the NOx storage catalytic converter. The NOx exhaustgas concentrations before and after the NOx storage catalytic converterare converted into NOx mass flows and the NOx storage efficiency isdetermined on the basis of these values.

DE 103 18 116 B4 describes a process to operate an internal combustionengine whereby a storage catalytic converter is regenerated. For thispurpose, the storage capacity is preset at the turning point of achronological signal curve of a NOx mass flow after the storagecatalytic converter.

DE 10 2006 055 238 A1 describes a method to operate an internalcombustion engine to determine the NOx storage capacity of a NOx storagecatalytic converter whereby exhaust gas is fed to two parallellyarranged NOx storage catalytic converters. Different amounts of loadsare generated for both storage catalytic converters. Subsequently, bothstorage catalytic converters are loaded simultaneously with nitrogenoxide until the signal of a NOx sensor signals that the absorptioncapacity of one of the storage catalytic converters has been exhausted.

The catalytic converter having NOx storage capacity can be a NOx storagecatalytic converter (NSK), or a Diesel oxidation catalytic converter(DOC). Inducing an exothermic process as well as providing an enrichmentstep are associated with an increased fuel consumption. Thus, such typeof measures are advantageously only applied if they are required toreduce the pollutants in the exhaust gas. The frequency of themonitoring events, in other words, of the respective calculations of acatalytic converter's nitrogen oxide storage capacity is thereforelimited. Should the NOx storage capacity of a catalytic converter beidentified as part of an on-board diagnosis (OBD), this diagnosis willonly be possible with this margin, or when accepting additional fuelconsumption, if additional monitoring events were requested. Forinstance, the diagnosis of a DOC using exothermic processes will beperformed as part of a particle filter regeneration. This generallyoccurs at an interval ranging from 500 km to 1,500 km of a distancecovered by the vehicle. For this type of monitoring, information on theDOC performance with respect to a reduction of the hydrocarbons and thecarbon monoxide in the exhaust gas will be achieved.

The diagnosis is performed more frequently for a nitrogen oxide storagecatalytic converter because such type of diagnosis generally occurs bymeans of an enrichment step. Thus, such type of event generally occursat a time interval between 1 km and 5 km of a distance covered by thevehicle.

This type of monitoring provides feedback on the nitrogen oxide storagecapacity of a catalytic converter. In turn, the nitrogen oxide storagecapacity of the catalytic converter is indicative of the efficiency withrespect to a reduction of the hydrocarbon or carbon monoxide in theexhaust gas.

Though for the new catalytic converter technologies in particular, asfor instance a passive nitrogen absorber (PNA), a DOC, or a passivelyrun NSK no or at least significantly less enrichment steps can bearranged. Accordingly, diagnostic events are only possible by means ofexothermic processes as part of the regeneration of the particle filteror by specially requested diagnoses, which in turn lead to an increasedfuel consumption.

The task of the present invention thus consists in providing for animproved method and an improved device of the initially mentioned type.

In the method according to the invention, the nitrogen storage capacityof a vehicle's catalytic converter will be determined. For this purpose,a concentration of nitrogen oxides will be measured downstream of thecatalytic converter. In at least one initial step, a concentration ofnitrogen oxides will be set in the exhaust gas at which the catalyticconverter absorbs nitrogen oxides. In at least one second step, aconcentration of nitrogen oxides will be set in the exhaust gas at whicha desorption of nitrogen oxides from the catalytic converter takesplace. To determine the nitrogen oxide storage capacity of a catalyticconverter, its behavior will be taken into account at least while thenitrogen oxides are desorbed.

In this manner, the catalytic converter's storage capacity can bedetermined qualitatively and indirectly even quantitively without anyspecially induced measures, which would lead to an increased fuelconsumption. Hereby, different effects or mechanisms can be used. In acatalytic converter having nitrogen oxide storage capacity, a nitrogenoxide saturation can be produced. The maximum storage capacity or storedquantities depend on the current partial NOx pressure in this case. Bylowering the partial NOx pressure, the maximum storage capacity or thestored nitrogen oxide quantiles will also be lowered and NOx will bedesorbed. Conversely, when increasing the partial NOx pressure, themaximum storage capacity or the stored quantity of nitrogen oxides andNOx will be reabsorbed by the catalytic converter. Therefore, byobserving a catalytic converter's behavior at least during thedeliberately induced desorption of nitrogen oxides, the nitrogen oxidestorage capacity of a catalytic converter can be inferred in an improvedmanner.

The information with respect to the nitrogen oxide storage capacity of acatalytic converter allows for a diagnosis of the catalytic converterwith respect to the HC/CO/NOx performance. Furthermore, using theinformation regarding the nitrogen oxide storage capacity of thecatalytic converter, the operating strategy of the combustion engine ora vehicle's motor can be optimized towards the exhaust system's currentstate. The vehicle can be a motor vehicle in particular or a commercialvehicle. Moreover, optimizing a strategy to desulfurize the catalyticconverter (DeSOx strategy) will be possible. It is assumed that thecatalytic converter has a NOx storage capacity, which changes as theexhaust system of the catalytic converter ages.

It is preferred in at least one first step to set a nitrogen oxideconcentration in the exhaust gas at which the catalytic converterabsorbs the nitrogen oxides until a saturation of the catalyticconverter with nitrogen oxides has been achieved.

It has been shown to be advantageous in this case, to realize an overrunmode of the vehicle after setting the catalytic converter's saturationwith nitrogen oxides, which will lead to a desorption of nitrogen oxidesfrom the catalytic converter. Hereby and based on the chronologicalsequence of the nitrogen oxides' desorption, the catalytic converter'snitrogen oxide storage capacity can be inferred. To determine the NOxstorage capacity, the above-mentioned mechanism can therefore beutilized that a NOx desorption exists in the vehicle's overrun mode orin the vehicle's operating state in which the raw emission of nitrogenoxides by the combustion engine is preferably quasi zero.

In accordance with another advantageous arrangement, a concentration ofnitrogen oxides will be set in the exhaust gas in a plurality of initialsteps whereby the catalytic converter absorbs nitrogen oxides. In aplurality of second steps, a concentration of nitrogen oxides will beset in the exhaust gas whereby the desorption of the nitrogen oxideswill be realized by the catalytic converter. Based on the majority ofthe stored or released amounts of nitrogen oxides in the initial stepsand in the second steps, the catalytic converter's nitrogen oxidestorage capacity can be inferred. In particular, the respective absorbedamounts of nitrogen oxides and the desorbed amounts of nitrogen oxidescan be accumulated separately from each other. The catalytic converter'sstorage capacity with respect to the nitrogen oxides can be inferredbased on the absorbed amounts and the desorbed amounts.

In accordance with another advantageous arrangement, respective initialgradients of a chronological sequence of concentrations of nitrogenoxides in the exhaust gas upstream of the catalytic converter, anddownstream of the catalytic converter are determined during at least oneinitial step. During at least one second step, respective secondgradients of a chronological sequence of the concentration of nitrogenoxides in the exhaust gas are determined upstream of the catalyticconverter and downstream of the catalytic converter. The catalyticconverter's nitrogen oxide storage catalyst can be inferred based on thegradients. Thus, a catalytic converter's nitrogen oxide storage catalystcan be inferred in particular when using a nitrogen oxide gradientidentification after the catalytic converter, particularly a nitrogenoxide storage catalyst.

Hereby an average value is preferably generated based on a plurality ofvalues from the first gradient and from a plurality of values from asecond gradient. The catalytic converter's nitrogen oxide storagecatalyst can then be inferred based on the average value. Such type ofmethod is feasible in a particularly simple and economical manner withrespect to the design of an appropriate control system or a controldevice of a vehicle to determine the catalytic converter's nitrogenoxide storage catalyst.

The nitrogen oxide storage capacity of a passive nitrogen oxide absorberof the vehicle and/or of an oxidation catalytic converter of the vehicleand/or of a passively operated nitrogen oxide storage catalyticconverter of the vehicle is preferably determined. In a passive nitrogenoxide absorber (PNA), the nitrogen oxides are stored or sorbed forinstance in the passive nitrogen oxide absorber's zeolite material,whereby the nitrogen oxides are not chemically bound, however.Therefore, enriching the air-fuel mixture to reduce chemically boundnitrogen oxides as part of a chemical reaction will be not be required.In fact, a thermic desorption of the nitrogen oxides occurs at thepassive nitrogen oxide absorber.

Likewise, an oxidation catalytic converter, particularly a dieseloxidation catalytic converter (DOC) has sorption capacities orabsorption capacities for nitrogen oxides for instance by using zeolitesas substrates of the catalytically effective substances of the oxidationcatalytic converter. In this case as well, a desorption of nitrogen cantake place without that the air-fuel mixture would need to be enriched,that is, without that an air ratio λ ratio greater than 1 would need tobe set.

Furthermore, a nitrogen storage catalytic converter can be operatedpassively whereby the nitrogen oxides are chemically bound to anappropriate material of the nitrogen oxide storage catalyst because thischemically bound NOx can also be thermally absorbed by raising thetemperature of the passively operated nitrogen oxide storage catalystfor instance.

Particularly in the case of such type of catalytic converters where noenrichment jump was set in the air-fuel mixture to obtain a reducedcontent of nitrogen oxide in the catalytic converter, the abovedescribed method will be applicable in a particularly advantageousmanner.

The device in accordance with the invention to determine a vehicle'scatalytic converter's nitrogen oxide storage capacity comprises a sensorto measure a concentration of nitrogen oxides in the exhaust gasdownstream from the catalytic converter. Furthermore, the devicecomprises a control device, which has been formed to set a concentrationof nitrogen oxides in the exhaust gas in at least one initial step atwhich the catalytic converter absorbs nitrogen oxides. In addition, thecontrol device is formed to set a concentration of nitrogen oxides inthe exhaust gas in at least one second step at which a desorption ofnitrogen oxides by the catalytic converter takes place. Furthermore, thecontrol device is formed to take the catalytic converter's behavior atleast into account when desorbing the nitrogen oxides to determine thecatalytic converter's storage catalyst of nitrogen oxide.

The described advantages and the preferred embodiments for the methodaccording to the invention apply for the device in accordance with theinvention and inversely.

The characteristics and combinations of characteristics described aboveas well as the characteristics and combinations of characteristicsdescribed below in the description of the figures and/or in the figuresby themselves are not only applicable in the respectively indicatedcombination but also in other combinations or by themselves withoutleaving the invention's scope. Thus, embodiments must be considered asbeing comprised by the invention and as disclosed that were notexplicitly shown or explained in the figures, but which follow fromseparate combinations of characteristics based on the explainedexplanations and which can be produced. Thus, embodiments andcombinations of characteristics can be considered as having beendisclosed that do not feature all the characteristics of an originallyformulated independent claim.

Additional advantages, characteristics, and details of the inventionresult from the claims, the subsequent description of preferredembodiments of the invention, as well as from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the chronological sequence of the nitrogen oxideconcentrations upstream of an oxidation catalytic converter anddownstream of an oxidation catalytic converter while a vehicle is inoverrun mode, which takes place subsequently to a saturation of theoxidation catalytic converter with nitrogen oxides;

FIG. 2 illustrates the chronological sequence of the NOx mass flowsupstream and downstream of the oxidation catalytic converter as well asthe respective totals of the absorbed or desorbed amounts of nitrogenoxides during the observed time interval; and

FIG. 3 illustrates the decelerated step response of the nitrogen oxideconcentration in the exhaust gas upstream of the oxidation catalyticconverter in the event of changes of the raw emissions of nitrogenoxide.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts schematically and partially exhaust system 10 of avehicle such as, for example, a passenger car or of a commercialvehicle. Exhaust system 10 comprises an exhaust gas system 12 in whichcatalytic converter 14 is arranged having nitrogen oxide storagecapacities. Catalytic converter 14 can be for instance an oxidationcatalytic converter, particularly a diesel oxidation catalytic converter(DOC). In exhaust gas system 12, particle filter 16, and/or an SCRcatalytic converter 18 can be connected in series after catalyticconverter 14. Furthermore, particle filter 16 may be particularly formedas an SCR-coated particle filter 16. To reduce nitrogen oxides in aselective catalytic reduction reaction in SCR-catalyst 18 (SCR=selectivecatalytic reduction), nitrogen oxides can be converted from the exhaustgas into water and nitrogen using ammoniac. For this purpose, an aqueousurea solution may be introduced in exhaust gas system 12 for instance bymeans of dosage device 20, whereby ammoniac is created from the ureasolution in the exhaust gas.

Downstream of catalytic converter 14 and in the present case upstream ofdosage device 20, sensor 22 is arranged in exhaust gas system 12 bymeans of which the concentration of nitrogen oxides in the exhaust gasdownstream of catalytic converter 14 can be measured. An appropriatecurve 24 in FIG. 1 illustrates the chronological sequence of thenitrogen oxides' (NOx) concentration in the exhaust gas downstream fromcatalytic converter 14. Additional curve 26 in FIG. 1 illustrates theraw nitrogen oxide emission, in other words, the concentration ofnitrogen oxides in the exhaust gas as they exist based on the operationof a combustion engine of the vehicle upstream of catalytic converter 14(not shown). The raw NOx emissions upstream of catalytic converter 14can be captured using a sensor or based on a model. A control devicesuch as in form of control device 28 of the vehicle will be used in thepresent case to determine the nitrogen oxide storage capacity ofcatalytic converter 14.

In the method to be illustrated with the aid of FIG. 1, a desorption ofnitrogen oxides of catalytic converter 14 is observed preferably in thevehicle's overrun mode to determine the NOx storage capacity ofcatalytic converter 14 because the NOx storage capacity of catalyticconverter 14 changes while exhaust system 10 ages, whereby the nitrogenoxide storage capacity of catalytic converter 14 reduces as the age ofexhaust system 10 increases.

In the method illustrated by means of FIG. 1, a saturation of catalyticconverter 14 with nitrogen oxides is produced in initial step 30. Thefact that at least a substantial saturation of the storage or catalyticconverter 14 exists, can be recognized by the fact that in the timeinterval of step 30, in which the NOx saturation is produced, the rawNOx emissions of the combustion engine (curve 26) are the same as thenitrogen oxide content in the exhaust gas downstream of catalyticconverter 14 (curve 24).

In point of time 34 applied into timeline 32 in FIG. 1, an operation ofthe combustion engine or the motor is set using control device 28 forinstance, in which the raw NOx emission is zero. This is advantageousbecause in that case the raw emission will be known without anymeasuring errors. Such type of condition occurs while the vehicle is inoverrun mode for instance. The overrun mode can also be supported by anelectrical motor of the vehicle so that any load requirements can be metwithout support from the combustion engine.

During a second step 36, which takes place immediately after point intime 34, catalytic converter 14 desorbs the nitrogen oxides.Accordingly, the concentration of nitrogen oxides in the exhaust gasdownstream of catalytic converter 14 (curve 24) during the time intervalfollowing point in time 34 is not zero; the concentration drops,however. The behavior of catalytic converter 14 during this desorptionof nitrogen oxides will be used to determine the nitrogen oxide storagecapacity of catalytic converter 14.

Preferably, the requirements to be met while doing so consist in thatthe NOx storage, that is catalytic converter 14, be saturated and thatthe temperatures before catalytic converter 14 and after catalyticconverter 14 neither increase nor drop drastically as it is advantageousthat the temperature gradient be not too high. The reason for this isthat the NOx storage capacity of catalytic converter 14 also depends onthe temperature in general. Thus, with an essentially constanttemperature, the desired desorption effect will not be superposed toosignificantly by the temperature effect.

In the event that exhaust system 10 was provided with an exhaust gasrecirculation, particularly a high-pressure exhaust gas recirculation,it is preferable that the exhaust gas recirculation be done without inoverrun mode. Otherwise nitrogen oxide emissions of the combustionengine would circulate in a circle and the raw emissions would drop moreslowly accordingly. However, as soon as the raw NOx emissions from theengine reach the value of zero (point in time 34), an exhaust gasrecirculation can be realized again, and the high-pressure exhaust gasrecirculation rate can be increased.

Furthermore, to reinforce the desorption, the exhaust gas mass flowthrough catalytic converter 14 can be modified by means of suitablemotor measures. In particular, the exhaust gas mass flow throughcatalytic converter 14 can be reduced to increase the measurementaccuracy when capturing the nitrogen oxide concentration by means ofsensor 22. For instance, changing the exhaust gas mass flow can berealized by appropriately setting the engine running speed, and/or byactivating a throttle valve, and/or by changing a high-pressure exhaustgas recirculation rate or low-pressure exhaust gas recirculation rate.

When these requirements are met, the nitrogen oxide concentration willbe preferably measured behind catalytic converter 14 using nitrogenoxide sensor 22. In the event that catalytic converter 14 is stillcapable at all to store nitrogen oxide, the nitrogen oxide concentrationbehind catalytic converter 14 will drop, in other words, downstream fromcatalytic converter 14, slower than before catalytic converter 14, thatis, than upstream of catalytic converter 14. Thus, a desorption ofnitrogen oxides by catalytic converter 14 takes place in overrun mode.

Preferably, the desorbed amount of nitrogen oxides in a modeled, thatis, expected nitrogen oxide amount will be adjusted. Such type of modelcan also indicate how the nitrogen oxide concentration in the exhaustgas downstream of catalytic converter 14 should reduce to zero should araw emission of nitrogen oxide be present. Thus, the model can forinstance take into account the subsiding of measurements of sensor 22 orthe existence of areas where the exhaust gas runs through less well inexhaust system 10, which accordingly can lead to a slowed down reductionof the nitrogen oxide concentration downstream of catalytic converter14. Moreover, the model takes into account the ageing of exhaust system10, particularly of catalytic converter 14.

If the actual measured course of the downstream nitrogen oxideconcentration reduction of catalytic converter 14 deviates from theexpected NOx decay curve, it is indicative of an appropriate change,particularly of a reduction, of the nitrogen oxide storage capacity ofcatalytic converter 14.

In particular, the desorbed nitrogen oxide amount, while in overrunmode, as well as the gradient of the nitrogen oxide decay curve, whilein overrun mode by means of a model suggests the absolute nitrogen oxidestorage capacity of catalytic converter 14. The nitrogen oxide storagecapacity of catalytic converter 14 used as a basis in a currently usedmodel will then be corrected upwards or downwards if the measurednitrogen oxide decay curve deviates from the target NOx decay curve.

Instead of observing the NOx saturation and the NOx desorption inoverrun mode, the amounts that accumulated during a plurality ofabsorption processes or desorption processes can be observed. This is tobe illustrated with reference to FIG. 2.

This variant is based on the finding that by lowering and increasing thepartial NOx pressure in the area of catalytic converter 14, smallabsorption processes and desorption processes occur continuously. Suchtype of variations of the partial NOx pressure occur in dynamic drivingsituations for instance when the raw NOx emissions of the combustionengine vary due to varying load requirements to the combustion engine.In this case, it is preferred to accumulate the respective NOx amountsseparately from each other. The total amount of nitrogen oxides that wasabsorbed, and the total desorbed amount of nitrogen oxides will then becompared with the quantities that are expected in accordance with amodel.

In FIG. 2 the NOx mass flow upstream or downstream of catalyticconverter 14 as a function of time t is applied on first ordinate 38,which is indicated on timeline 32. Initial curve 40 thereforeillustrates the NOx mass flow upstream of catalytic converter 14accordingly, and second curve 42 illustrates the NOx mass flowdownstream from catalytic converter 14. In a plurality of initial steps44, the emissions before catalytic converter 14 are higher than theemissions after catalytic converter 14. Accordingly, an absorption ofnitrogen oxides occurs. Analogously, a desorption of nitrogen oxides bycatalytic converter 14 takes place in a plurality of second steps 46.This is the case when the emissions are higher downstream of catalyticconverter 14 than the emissions upstream of catalytic converter 14.

In another diagram in FIG. 2, the respective totals of the accumulatedamounts of nitrogen oxides over time t are applied to additionalordinate 48. Curve 50 illustrates the accumulation of the absorption andcurve 52 illustrates the accumulation of the desorption. Time t in turnis applied on timeline 32 in the second diagram in FIG. 2. Within theobserved time interval, difference 54 between the added up absorptionamounts and the added up desorption amounts results. This difference 54will be compared with an index value and based on the comparison, aconclusion will be made with respect to the nitrogen oxide storagecapacity of catalytic converter 14.

To determine the nitrogen oxide amounts as correctly as possible, it isimportant that the NOx concentrations before catalytic converter 14 orafter catalytic converter 14 be known with low margins of error. In theevent that measuring errors do exist, these errors before catalyticconverter 14 and after catalytic converter 14 should tend to go into thesame direction. The existence of errors can be caught for instance byobserving conditions in which a storage of nitrogen oxides in catalyticconverter 14 does not occur. In that case, the measuring values upstreamof catalytic converter 14 or downstream of catalytic converter 14 shouldmatch or the modeled value upstream of catalytic converter 14 shouldcorrespond to the value downstream of catalytic converter 14 as measuredwith sensor 22. Furthermore, the nitrogen oxide concentrations shouldexist at the correct point in time. For this, the values of sensorsbefore catalytic converter 14 or after catalytic converter 14 (or thevalues provided by the raw emission model and the values provided bysensor 22 after catalytic converter 14) should be geared to each otherchronologically. The appropriate procedures are known to the expert.

To determine the absorption processes and desorption processes, thenitrogen oxide absorption and the nitrogen oxide desorption of catalyticconverter 14 are continuously ascertained. This takes place by means ofa subtraction of the nitrogen oxide mass flows before catalyticconverter 14 and the nitrogen oxide mass flows after catalytic converter14 from each other. NOx sensor 22 has been provided to determine the NOxmass flow downstream of catalytic converter 14. An emission model canalso be used to determine the NOx mass flow before catalytic converter14.

The nitrogen oxide absorption mass flows and the nitrogen oxidedesorption mass flow are accumulated or added up separately from eachother. Furthermore, the nitrogen oxide of the nitrogen oxide rawemissions of the combustion engine will be accumulated. In addition, aspecific value will be determined, which will be representative for theaveraged nitrogen oxide gradient upstream of catalytic converter 14,which in other words indicates the averaged increase of a curve, whichrepresents the nitrogen oxide raw emissions of the combustion engine.Such type of specific value can be calculated or indicated in ppm_(NOx)per second in particular. Accordingly, a high specific value exists inthe event of significant changes of the raw emissions. In contrast, alow specific value indicates less strong variations of the nitrogenoxide raw emissions.

Additionally, the difference between the total absorbed amounts ofnitrogen oxides and the total desorbed amounts of nitrogen oxidesdepends on the temperature profile and on the raw emission during theevaluation period. For instance, the nitrogen oxide storage capacity ofcatalytic converter 14 generally increases when the temperature ofcatalytic converter 14 drops. Accordingly, the accumulated amount ofabsorbed nitrogen oxide is larger than the accumulated amount ofdesorbed nitrogen oxide. Conversely, an increased temperature ofcatalytic converter 14 will cause the accumulated desorption to begenerally larger than the accumulated absorption.

Difference 54, which exists at the end of the observed evaluation periodwill be compared with an expected, modelled difference. If difference 54is greater or smaller than the expected difference, it could suggest adrift of the emission model or of sensor 22. This can be compensated byan appropriate new calibration of the emission model or of sensor 22. Inthe event that no such drift exists, the NOx storage capacity ofcatalytic converter 14 can be inferred based on difference 54.

The diagnosis will be preferably performed while observing a past timeinterval. Thus, it can be assured that pre-determined fringe conditionsexisted within the time interval. It can be provided that one of thesefringe conditions be that the NOx storage, in other words, catalyticconverter 14, was sufficiently saturated with nitrogen oxides during theobserved, past time interval. For instance, a saturation of a minimum of80 percent can be allowed for. Moreover, a time interval is preferablyobserved in which the temperature was sufficiently stable, and waswithin a permitted range. For this, it can be considered for instancewhether in case of a temperature change of the exhaust gas upstream fromcatalytic converter 14 no or at the most a slight temperature changetook place downstream of catalytic converter 14. Fixing this fringecondition in turn is founded on the desorption's temperature dependency.

As an additional fringe condition, it can be provided that the specificvalue for the nitrogen oxide gradient upstream from catalytic converter14 was within a pre-determined range. This range should not be toosmall. Otherwise the absorption effects and the desorption effects willbe very low so that they cannot be captured very well meteorologically.By contrast, if the range is too wide, the tolerances of the emissionmodel and the tolerances of sensor 22 can become too great. Furthermore,it can be checked whether the difference between the absorption and thedesorption of the past time interval is plausible as a fringe condition.This can be checked in particular while consulting a model.

To assess the diagnosis, the amount of the accumulated absorption andthe amount of the accumulated desorption will be compared with themodelled absorption and the modelled desorption. In the event that thedifference of the accumulated absorption total and the accumulateddesorption total is lesser than the modelled difference or the modelledtotal, it is a sign for a reduced nitrogen oxide storage capacity ofcatalytic converter 14. Thus, based on the absorption quantities and thedesorption quantities, the absolute nitrogen oxide storage capacity ofcatalytic converter 14 can be inferred by means of a model. The modelpreferably comprises a target value for the absorption amount and atarget value for the desorption amount for the provided fringeconditions in the provided time interval. These target values constitutea function of the temperature, the specific value of the NOx gradient,of the exhaust gas mass flow, and the total of the nitrogen oxide rawemissions. The currently modelled nitrogen oxide storage capacity ofcatalytic converter 14 can be corrected upwards or downwards when themeasured absorption amount and the measured desorption amount deviatesfrom the target absorption amount and the target desorption amount.

An additional possibility of taking into account the desorption of thenitrogen oxides by catalytic converter 14 depending on the concentrationof nitrogen oxides in the exhaust gas to determine the nitrogen oxidestorage capacity of catalytic converter 14, is to be illustrated whilereferring to FIG. 3, where the nitrogen oxide concentration is appliedin a diagram on ordinate 56, and where time t is applied in turn ontimeline 32. Initial curve 58 represents the course of the raw emissionsas a function of time, and second curve 60 represents the concentrationof the nitrogen oxides in the exhaust gas downstream of catalyticconverter 14, which is recorded by means of sensor 22. In this case aswell, the effect is used that by lowering or increasing the partialpressure of the nitrogen oxide by means of variations as they arecustomary in dynamic driving situation, resulting continuous and smallabsorption processes and desorption processes occur. However, due tothese absorption processes and desorption processes the nitrogen oxideconcentration after catalytic converter 14 changes more slowly than thenitrogen oxide concentration before catalytic converter 14. Thecorresponding step response, in other words, the change of the nitrogenoxide concentration downstream of catalytic converter 14, which is theresult of a corresponding change of the nitrogen oxide concentrationupstream of catalytic converter 14, will be compared with a stepresponse as expected from a model. In particular, the absolute nitrogenoxide storage capacity of catalytic converter 14 can be inferred using astatistic analysis of several of such step responses by means of amodel.

Preferably, one proceeds in the following manner to determine thespecific value for the NOx gradient. Initially, a signal noise ofnitrogen oxide sensor 22 after catalytic converter 14 as well as the(optional) nitrogen oxide sensor upstream of catalytic converter 14 willbe averaged out. The gradient or the increase of nitrogen concentrationsupstream of catalytic converter 14 will be determined in ppm_(NOx) persecond for instance. As a basis for this, a nitrogen oxide sensorupstream of catalytic converter 14 (not shown) or a nitrogen oxide rawemission model will be used. Furthermore, the gradient or the nitrogenoxide concentration increase downstream of catalytic converter 14 willbe preferably ascertained in ppm_(NOx) per second for which nitrogenoxide sensor 22 serves as a basis.

Subsequently, an average value will be formed for a defined time periodof a minimum of 100 seconds for instance based in the totals of nitrogenoxide gradients before catalytic converter 14 and after catalyticconverter 14. This average value is a specific value that isrepresentative of the nitrogen oxide gradient.

For instance, according to FIG. 3, an absorption of nitrogen oxide takesplace in catalytic converter 14 during initial step 62. This can berecognized by a significant increase of the raw emissions (curve 58)being followed by a significantly slower increase of the nitrogen oxideconcentration downstream of catalytic converter 14 (curve 60).Conversely, in second step 64, a nitrogen oxide desorption by catalyticconverter 14 occurs where the nitrogen oxide raw emission (curve 58)rapidly drops accordingly, while the nitrogen oxide concentrationupstream of catalytic converter 14 decreases more slowly. Due to theseslower step responses of nitrogen oxide concentrations downstream ofcatalytic converter 14, which is recorded by means of sensor 22, theexistence of an absorption during a plurality of initial steps 62 or ofa desorption during a plurality of second steps 64 can be inferred.

Also, when recognizing the nitrogen oxide gradient downstream ofcatalytic converter 14, which exhibits a nitrogen oxide storagecapacity, the diagnosis is preferably performed on a past time interval.In this case as well, it can be provided as a fringe condition that theNOx storage or catalytic converter 14 was sufficiently saturated withnitrogen oxides in the past time interval. Furthermore, it is preferablydetected that the exhaust gas temperature in the past time interval wassufficiently stable and within the permitted range. The specific valuefor the nitrogen oxide gradient was preferably upstream of catalyticconverter 14 within a pre-determined range. If this range was too small,the tolerances and the noise made by the sensors dominated excessively.In contrast, if the specific value was too high, the tolerances of theemission model and the sensors' tolerances were too high.

By means of the specific value, which can be ascertained by means of theaverage values of the totals of the NOx gradients before catalyticconverter 14 and after catalytic converter 14, the absolute nitrogenoxide storage capacity of catalytic converter 14 can be inferred using amodel. The model preferably has a target value for the specific value ofthe nitrogen oxide gradient after catalytic converter 14 for theprovided parameters in the provided time interval. This is a function ofthe temperature, the exhaust gas mass flow, the total of the nitrogenoxide raw emission and of the nitrogen oxide gradient before catalyticconverter 14, however. The currently modelled nitrogen oxide storagecapacity of catalytic converter 14 will be corrected upwards ordownwards if the measured specific value of the nitrogen oxide gradientdeviates from the target value.

The knowledge of the nitrogen oxide storage capacity of catalyticconverter 14 can be used for the operating mode of exhaust system 10.For instance, the engine's raw nitrogen oxide emission can be adaptedvia the engine control unit. Furthermore, an enrichment step dosagestrategy and/or a urea dosage strategy can be adapted or heatingmeasures of the exhaust gas can be taken.

Moreover, based on the storage capacity of catalytic converter 14, thecurrent performance of catalytic converter 14 in respect of a reductionof the content of hydrocarbon (HC), carbon monoxide (CO), and nitrogenoxides (NOx) in the exhaust gas can be inferred. Thus, a diagnosis ofcatalytic converter 14 according to OBD will be possible. For thediagnosis, an error can be reported in the event of a lower deviation ofa critical nitrogen oxide storage capacity or storage amount. Theconsequence of this would be that an engine control light would beactivated. In addition, or alternatively, additional diagnostic measurescould be initiated to validate the result for instance. Such type ofvalidation measures can be realized based on the state of the art.

Also, the current Sulphur concentration in the fuel and the Sulphur loadof catalytic converter 14 can be inferred based on the change of thenitrogen oxide storage capacity of catalytic converter 14 within a timeinterval or interval in which a desulfurization of catalytic converter14 takes place. Therefore, an optimization of a DeSOx strategy will bepossible, that is, a desulfurization strategy of catalytic converter 14.In particular, the interval or the time interval between two DeSOxmeasures and the DeSOx intensity can be optimized. The DeSOx intensityis particularly expressed in a depth of the enrichment step, in otherwords, in the extent or intensity of the enrichment of the air-fuelmixture, in the duration of the enrichment step and in the number ofenrichment steps. The desulfurization of catalytic converter 14 can berealized particularly under increased temperatures, for instance as partof a regeneration of particle filter 16, by means of appropriateenrichment steps.

REFERENCE CHARACTERS

-   10 Exhaust system-   12 Exhaust gas system-   14 Catalytic converter-   16 Particle filter-   18 SCR catalytic converter-   20 Dosage device-   22 Sensor-   24 Curve-   26 Curve-   28 Control device-   30 Step-   32 Timeline-   34 Point in time-   38 Ordinate-   40 Curve-   42 Curve-   44 Step-   46 Step-   48 Ordinate-   50 Curve-   52 Curve-   54 Difference-   56 Ordinate-   58 Curve-   60 Curve-   62 Step-   64 Step

1.-10. (canceled)
 11. A method to determine a nitrogen oxide storagecapacity of a catalytic converter of a vehicle in which a concentrationof nitrogen oxides is measured in an exhaust gas downstream of thecatalytic converter, comprising the steps of: in at least one firststep, setting a concentration of nitrogen oxides in the exhaust gas atwhich the catalytic converter absorbs nitrogen oxides; in at least onesecond step, setting a concentration of nitrogen oxides in the exhaustgas at which a desorption of nitrogen oxides by the catalytic convertertakes place; and determining the nitrogen oxide storage capacity of thecatalytic converter by considering a behavior of the catalytic converterat least during the desorption of nitrogen oxides.
 12. The methodaccording to claim 11, wherein the concentration of nitrogen oxides isset in the exhaust gas at which the catalytic converter absorbs nitrogenoxides such that a saturation of the catalytic converter with nitrogenoxides occurs.
 13. The method according to claim 12, wherein after thesaturation, an overrun mode of the vehicle is realized, and whereinbased on a chronological sequence of the desorption of nitrogen oxides,the nitrogen oxide storage capacity of the catalytic converter isinferred.
 14. The method according to claim 11, wherein in a pluralityof first steps, the concentration of nitrogen oxides is set in theexhaust gas at which the catalytic converter absorbs nitrogen oxides,and in a plurality of second steps, the concentration of nitrogen oxidesis set in the exhaust gas at which the desorption of nitrogen oxides bythe catalytic converter takes place, wherein based on a stored and areleased amount of nitrogen oxides via the plurality of first steps andsecond steps, the nitrogen oxide storage capacity of the catalyticconverter is inferred.
 15. The method according to claim 11, whereinrespective initial gradients of a chronological sequence of theconcentration of nitrogen oxides in the exhaust gas are determinedupstream of the catalytic converter and downstream of the catalyticconverter during the at least one first step, wherein respective secondgradients of a chronological sequence of the concentration of nitrogenoxides in the exhaust gas upstream of the catalytic converter anddownstream of the catalytic converter are determined during the at leastone second step, wherein based on the gradients the nitrogen oxidestorage capacity of the catalytic converter is inferred.
 16. The methodaccording to claim 15, wherein an average value is created from amultitude of totals of the first gradient and from a multitude of totalsof the second gradient, wherein the nitrogen oxide storage capacity ofthe catalytic converter is inferred on a basis of the average value. 17.The method according to claim 11, wherein the nitrogen oxide storagecapacity of a passive nitrogen oxide absorber of a vehicle isdetermined.
 18. The method according to claim 11, wherein the nitrogenoxide storage capacity of an oxidation catalytic converter of a vehicleis determined.
 19. The method according to claim 11, wherein thenitrogen oxide storage capacity of a passively operated nitrogen oxidestorage catalytic converter of a vehicle is determined.
 20. A method todetermine a nitrogen oxide storage capacity of a catalytic converter ofa vehicle with a sensor to measure a concentration of nitrogen oxides inan exhaust gas downstream of catalytic converter, comprising the stepsof: in at least one first step, setting a concentration of nitrogenoxides in the exhaust gas at which the catalytic converter absorbsnitrogen oxides by a control device; in at least one second step,setting a concentration of nitrogen oxides in the exhaust gas at which adesorption of nitrogen oxides by the catalytic converter takes place bythe control device; and determining the nitrogen oxide storage capacityof the catalytic converter by considering a behavior of the catalyticconverter at least during the desorption of nitrogen oxides by thecontrol device.